main toolbar rework (integrate player dialog)

[guiv42]

see #533 @helge17: I would like to have your opinion before merging, it's always highly subjective when it comes to user interface. This evolution targets a version 2.0 and is inspired by 2.0beta fork, but I'm following a more conservative path.

Timestamp display is the most challenging evolution:

• font is configurable, just click on timestamp
• most probably a specific setting will be needed for font on macOS, could you please have a look?
• foreground & background colors are configured for some skins (see skin.prop files)
• timestamp zero is always displayed when not playing, it's intentional. 2.0beta always displays a timestamp, but when not playing the value becomes erroneous after the first "repeat close" (could not try when playing, I can't make it work). Parsing the whole song is necessary to compute a correct timestamp, it's done when player starts but cannot be done permanently.
• I hope it will work correctly on macOS, I have struggled to get something working correctly in all configs [Linux,Windows]*[SWT,JFX]